Compressors are employed within air conditioning systems for compressing refrigerants and circulating them through a condenser and an evaporator. In the condenser the gaseous refrigerant is condensed at high pressure. The liquid refrigerant enters the evaporator where the liquid evaporates and removes heat from the air conditioned space. The vapor is then returned to the suction or inlet side of the compressor to repeat the cycle.
Conventionally, some lubricating oil from the compressor invariably escapes to the refrigerant circuit. To insure proper lubrication, this lubricant must be returned to the compressor oil sump. The return of this lubricant by normal circuit gas flow is accomplished if circuit factors such as length and elevation of circuit and gas flow are favorable. Conventionally, when the above mentioned circuit conditions are not favorable, an oil separator (located at or near the compressor discharge) separates the oil from the refrigerant. The accumulated oil from the separator is then returned by various means into the compressor inlet gas stream where it returns to the compressor oil sump. The efficiency and means of the oil return to the compressor oil sump is dependent on internal compressor design.
For example, U.S. Pat. No. 4,912,937 (Nakamura 1990) discloses an air conditioning system with an oil separator which shunts oil discharged from the compressor back to the suction end of the compressor. A switch in the separator signals a solenoid when the accumulated oil reaches a predetermined maximum. The solenoid opens the drain line to allow the return of lubricating fluid to the compressor inlet.
U.S. Pat. No. 3,280,576 (Endress 1966) discloses a complex refrigeration system which includes a method and apparatus for lubrication that is designed to prevent oil foaming due to oil sump pressure changes when system is operated after a shut-down. The system provides for an oil pump to maintain positive oil pressure. Thus, Endress discloses a lubricating system that, to prevent foaming, isolates the sump oil from the remainder of the system during shut-down periods.
U.S. Pat. No. 4,180,986 (Shaw 1980) discloses a constant operation refrigeration system employing a compressor by-pass to reduce compressor capacity. Evaporator conditions control the reduced capacity cycle and at the same time call for a reduced oil flow by engaging a solenoid which places a restrictor in the oil supply line to the compressor. Upon return to full capacity operation, the solenoid circuit returns the lubrication system to full flow. The primary function of such a system is to conserve oil during reduced system loads for use when increased system loads are required. Evaporator pressures reflect refrigeration load conditions and are thereby used to signal reduced compressor capacity and reduced oil flow cycle.
None of the present systems provide "fail-safe" lubrication under conditions peculiar to aircraft air conditioning systems, specifically: (1) limited compressor weight (five to thirteen pounds); (2) limited compressor cooling capabilities (one to three tons); (3) relatively long circuits (generally exceeding 15 feet between the compressor/condensor and the evaporator); (4) extreme elevation variations; (5) variable gravity conditions (both positive and negative "g" loads); (6) use of hose instead of pipe; (7) limited compressor lubricating oil capacity (generally three to ten ounces). These factors require long reach aircraft air conditioning systems with only a few ounces of lubricating oil, a compressor mounted high relative to other circuit components, utilized under variable gravitational conditions. Moreover, the use of hosing instead of pipe produces a greater potential for residue and traps. These conditions demand that almost no oil be permitted to enter the circuit because such escaped oil will not readily return to the compressor oil sump. An oil separator must be employed in the gas discharge line adjacent to the compressor to provide for the return of the separated oil back to the compressor crankcase by the most direct means.